I am a bit new to gas engines.

I have bought a NGH 26GT which I have mounted inverted in my new bipe. I have been conducting test/break-in runs in the garden and a couple of times the plug cap has fallen off. It has a glow plug thread miniature spark plug and the cap has a small spring inside which would tend to push it off.

The instructions are a bit scanty. Am I not pushing it on hard enough or ought I to squeeze the cap slightly to get a better grip? Or do you twist it to lock once it is on?

Suggestions gratefully received!